RFR [15] 8241988 DatagramSocket incorrectly caches the first set of socket options

[Chris Hegarty]

DatagramSocket incorrectly caches the first set of socket options that
it encounters (through an invocation of supportedOptions()),
subsequently returning that set of options regardless of what the
particular datagram socket impl actually supports.

Webrev:
  https://cr.openjdk.java.net/~chegar/8241988/webrev/
JIRA:
  https://bugs.openjdk.java.net/browse/JDK-8241988

This issue has been filed so as to extract the long(ish) standing issue
from the work being done to support JEP 373 [1]. The code changes in 
this review are only slightly different to the changes being proposed in
the review for 373 [1], so can be merged will little effort.

-Chris.

[1] https://mail.openjdk.java.net/pipermail/net-dev/2020-April/013748.html